1. Field of the Invention
This invention relates to a connector for a coaxial cable and in particular to a two-piece connector which upon assembly becomes a one-piece connector which provides a connection which is completely shielded and thus remains leakproof to electromagnetic radiation over time.
2. Description of the Prior Art
Coaxial cable (FIGS. 1a and 1b ) consists of a centrally located conductor (typically copper) 1 surrounded by a first dielectric insulator 2, which forms an annular ring of an approximately uniform thickness around the centrally located conductor 1. The outer surface of the dielectric insulator 2 is covered by an outer conductor (typically a uniformly circularly braided conducting wire such as aluminum) 4 which serves as a ground shield and which in turn is covered by a second dielectric layer 5 (sometimes called the outside or outer insulation layer). Originally, the outer (shielding) conductor was a single layer of uniformly circularly braided conducting wire 4. More recently a third layer of conductive material 3 (typically a relatively thin covering such as a foil of the same conductive material as the wire braid), shown in FIG. 1b, has been added under the wire braid outer conductor 4 but outside the first layer of dielectric insulation to provide additional shielding. Conductive material 3 can be bonded to first dielectric 2 or can be unbonded, and can be applied in various thicknesses which are known as single, double, and triple foil cable. Outer conductor 3, as noted above the layer of uniformly circularly braided conducting wire, covers this foil. Outer conductor 4 is typically a braid which is manufactured in various braid coverage percentages, i.e., 40%, 67%, and 90%. Second dielectric layer 5 surrounds the outer conductor 4 (FIGS. 1a and 1b).
Absent defects in the cable, the industry has accepted that coaxial cable alone provides a very good means for shielding electrical signals from their surrounding electromagnetic environment, particularly at signal frequencies above 5 MHz.
Coaxial cables are commonly used to transmit video signals. To ensure a clean, clear picture on a television set, it is important to avoid interference between the electrical signal carried through the coaxial cable and the surrounding electromagnetic environment.
Any loss of shielding when connecting one coaxial cable to another by means of a coaxial cable connector can cause interference between signals being conducted in and transmitted outside the cable. Connectors for coaxial cable have evolved over time and many different structures have been tried to connect coaxial cables while maintaining the integrity of both the insulation and the shielding of the coaxial cable and of the connector. Each prior art structure has some performance or cost drawback.
While coaxial cables are used in many industries, a particularly important use is in the telecommunications industry for transmitting television signals from a receiving antenna or cable television source to television sets. While coaxial cable is a good means for transporting the television signal, whenever there is a termination of the coaxial cable requiring a connector (such as connecting the coaxial cable to a main cable line, connecting the coaxial cable to a customer's point of service, or just to lengthen a previously installed cable) the cable television industry has found that the television signal carried on the central conductor in a coaxial cable will egress as well as receive outside signals when there is a gap between the shielding of the coaxial cable and the connector. This loss of shielding integrity allows external signals to be picked up by the central conductor in the coaxial cable and to interfere with the cable television signal and also allows the cable television signal to leak out of the coaxial cable.
In 1935 the F.C.C. assigned a frequency spectrum to be used for transmitting television signals. The frequency band from 50 MHz to 88 MHz contains channels 2 through 6 and the frequency band from 174 MHz to 216 MHz contains channels through 13 for a total of 12 VHF channels. State of the art cable systems have up to 88 channels and cover frequency spectrum from 5 MHz to 550 MHz. This is allowed only if the television signals remain inside the coaxial cable. If the signals are allowed to escape the coaxial environment, i.e. be retransmitted from faulty connectors, they can and do interfere with sensitive frequency bands such as those utilized by, for example, police and fire department radios, aircraft navigation systems, and marine and aircraft distress signals.
Because there is normally a timing delay between signals sent over cable television lines when compared to signals received directly from an antenna source, two out-of-phase signals, a strong signal and a weak signal, are received by the television tuner. The presence of two such signals causes what is commonly known in the industry as "ghosts."
A solution is needed to eliminate "ghosts" created as a result of interference between television signals sent via coaxial cable from a cable television source and television signals which are transmitted through the environment by television stations (and are available in most cities and towns merely by an antenna hookup).
Apart from a few exceptions, experience has shown that problems which cable customers experience having to do with interference or "ghosts" can be traced to connector failure. A connector is said to have "failed" when interference problems associated with signal leakage are eliminated by the replacement of that particular coaxial cable connector. While the connectors individually cost less than fifty cents per unit, the cost of sending a technician to locate and identify a customer problem or replace connectors due to normal maintenance or system expansion can amount to $30.00 or more per connector unit.
This problem has been identified in the cable television industry for a number of years. Research has recently been undertaken to compare the various connectors available on the market and their performance compared with each other over time. Preliminary results of this ongoing study indicate that each connector examined exhibits a maximum level of performance at the time of assembly and installation. This performance degrades measurably with time until at some point the performance is so low that the connector is deemed to have "failed."
Historically, the first connectors for coaxial cables (illustrated in FIG. 2a in an exploded view) were two piece connectors generally referred to in the industry as F-connectors. Connector 8, illustrated in FIG. 2a, is illustrative of a typical F-connector which is comprised of free-spinning nut 9 which is retained and integrated at one end of hollow post 10 by collar 11. Barb 12 is provided at the opposite end of post 10. The second piece of the two pieces is metal sleeve 13 which, when crimped in place around outside insulator 5 of a coaxial cable which has been pressed onto the hollow post 10, holds the connector on the end of the coaxial cable. The inside diameter of the opening in post 10 is slightly larger than the outside diameter of first dielectric 2. When post 10 is installed on a coaxial cable, the dimensions of barb 12 and thickness of post 10 in barrel portion 14 is such that barb 12 and barrel portion 14 are positioned between first dielectric 2 and outer conductor 4.
These pieces are assembled by the following steps illustrated in FIGS. 2b-2d. As illustrated in FIG. 2b, typically outer insulator 5 is stripped off for a distance of 1/2 an inch, then the exposed outer braid conductor 4 is folded back along the outer insulation (FIG. 2c). Then the first dielectric 2 is stripped away for a distance of 3/8" exposing the center conductor 1 (FIG. 2d). Metal crimp sleeve 13 is placed over the end of the coaxial cable. Then, the end of hollow post 10 having barb 12 is slipped over first dielectric 2 covered with the third layer of conductive material 3, typically aluminum foil, paying careful attention to leave the third layer of conductive material 3 intact and undamaged. Post 10 is forced down along first dielectric 2 until it is stopped by end 15 of collar 11 meeting the end of outer insulation layer 5 and braid outer conductor 4. Post 10 is forced down between the third layer of conductive material 3 covering the outside of first dielectric insulator 2 and outer conductor 4 which is inside of second dielectric layer 5. Metal sleeve 13 which was first put on the end of the cable is then slipped over the outside of end connector where post 10 with barb 12 has been stopped and is then crimped in place. Second dielectric layer 5 and outer conductor 4 are trapped between crimp sleeve 13 and post 10, which acts as a mandrel, and this prevents second dielectric layer 5 from becoming elliptical or misshaped.
Historically, this crimping has been done in many different ways. One way was to crimp sleeve 13 as mechanical wire connectors are crimped, at the center (i.e., with pliers or a standard wire crimping tool), relying on the work-hardening of the material of the crimped sleeve 13 to maintain the inward force on the outside insulation 5, forcing outer conductor 4 of the cable onto barb 12 of post 10 and relying on the strength of post 10 to not crush during the crimping process.
In a second crimping technique which has been used oversized sleeve 13 is crimped into two loops, one around the cable, the other smaller one off to one side consisting of the excess circumference of the sleeve 13 not needed to crimp the loop around the cable. This prevented damage to dielectric insulator 5 by direct crimping. Work-hardening of the sleeve material provided the crimping force. Proper or improper crimping in this manner would often cause the sleeve 13 to break at its point of greatest bending, releasing the tension thus causing the connection to fail.
In yet another method, metal sleeve 13 is crimped on post 10 and barb 12 using a hex-patterned crimp. The general idea of this method of attachment is to distribute the crimping force somewhat uniformly around outer insulation layer 5 maintaining a mechanically tight connection. A special hex-crimping tool is used to make this crimp. Unfortunately, this method did not solve the problem of uniform shielding as pressure was concentrated on the six flats of the hex while the six points had little or no pressure (FIG. 2e).
While at the time of assembly this connection seemed to be quite tight and efficient, over time the metal of the sleeve 13 which had been crimped relaxed slightly and insulation 5 which had been captured by crimping flowed to a point of lower stress thereby making the connection loose.
A one-piece connector, of which connector 17 illustrated in exploded view in FIG. 3 is an example, has also been manufactured and used. It differs from two-piece connector 8 only in that the metal sleeve 18 which was crimped over the coaxial cable is also fixed to post 19, whereas in two-piece connector 8 metal crimp sleeve 13 is loose. Connector 17 is provided commercially with nut 20 installed on post 19 and metal sleeve 18 is pressed into place on post 19 to form the completed, assembled unit as illustrated in FIG. 4 in partial cut-away fashion. One problem with a connector such as connector 17, in addition to the problem with loosening after a period of time after assembly, was that during assembly of connector 17 on to a coaxial cable, the insertion of post 19 between conductive foil 3 covering first dielectric insulator 2 and the wire braid outer conductor 4 inside the outside insulation layer 5 could not be observed If during installation, as post 19 was being inserted into the cable the foil was wrinkled or torn a faulty connection could result.
A product developed by the Raychem Corporation to attempt to address the above-noted problems is generally called an EZ-F type connector. The EZ-F connector as manufactured by Raychem consists of four pieces in a single assembly, an example of which is illustrated in FIG. 5 (each piece illustrated in cross section) and the assembly indicated by reference character 23. The individual parts of connector 23 are post 24, compression ring 25, retaining nut 26, and outside piece 27. As illustrated in FIG. 6, outside piece 27 encloses the completed assembly The post 24 is positioned within outside piece 27 and receives the end of the stripped coaxial cable. Compression ring 25, composed of a plastic material, is placed between post 24 and retaining nut 26. As best illustrated in FIG. 6, retaining nut 26 holds the assembly together and prevents compression ring 25 and post 24 from coming out of outside piece 27. The F-connector type female threads 28 in the front of outside piece 27 are of such a diameter that post 24 cannot slip through that space. F-connector type female threads 28 in the front of outside piece 27 are 3/8".times.32 TPI threads, the type normally used in coaxial connectors. As generally commercially sold, connector 23 is completely assembled, with retaining nut 26 holding compression ring 25 and post 24 within outside piece 27.
After the stripped coaxial cable (with wire braid outer conductor 4 folded back over outside insulation layer 5 for approximately one-eighth inch) is inserted into an assembled connector 23, a tool is utilized to lock connector 23 on to the end of the coaxial cable. This tool threads into connector 23 forcing compression ring 25 to plastically deform into the annular open space 29 of post 24 to clamp and hold outside insulation layer 5 of the coaxial cable, and the wire braid outer conductor 4 in annular space 29 of post 24. In contrast to a one piece connector such as connector 17 (illustrated in FIGS. 3 and 4), post 24 is nickel plated brass and performs very efficiently when studied in comparison with other connectors. FIG. 6 illustrates connector 23 which has been crimped onto the end of a coaxial cable For ease of understanding, a highly enlarged cross section taken along lines 7--7 is illustrated in FIG. 7. One of the problems which plagued that type of connector that still exists with the EZ-F type connector in that the insertion of the coaxial cable into the assembled connector 23 is blind, i.e., the assembler cannot see how post 24, which is being forced between foil 3 covering first dielectric insulator 2 and wire braid outer conductor 4 inside outside insulation layer 5 is progressing. Thus post 24 can wrinkle and tear foil 3 covering first dielectric insulator 2 without the assembler realizing it, thereby creating a faulty connection.
Another manufacturer, LRC Augat, has provided a coaxial cable connector which is generally referred to as a Snap-N-Seal connector. A connector of this type is illustrated in FIGS. 8 and 9, and indicated by reference character 30. A similarly constructed connector is also illustrated in U.S. Pat. No. 4,834,675, issued May 30, 1989. As will be best appreciated by reference to FIG. 9, connector 30 contains a free-wheeling nut 31 and a centrally located hollow post 32 and plastic sleeve 33, which locks in place in outer casing 34 upon final assembly Outer casing 34 is, however, much larger in diameter than any of the other parts of any of the connectors described above which contact wire braid outer conductor 4.
During assembly, the cable is inserted through plastic sleeve 33 with shoulder 35 of sleeve 33 away from the end (FIG. 9). Then connector 30 is pushed on to the cable. Plastic sleeve 33 is then pressed into outer casing 34, securing plastic sleeve 33 in outer casing 34 and also pressing the wire braid outer conductor 4 which is extending out of the end of the coaxial cable inside outer casing 34 against the casing body. Once plastic sleeve 33 has been inserted, it is held there elastically by locking depression 36 (FIG. 8) in outer casing 34 near the left hand side (as viewed in FIG. 8) of outer casing 34. Locking depression 36 matches with locking projection 37 (FIG. 8) on plastic sleeve 33 to cause sleeve 33 to be permanently locked in place in an elastically compressed state. The force used to introduce plastic sleeve 33 into outer casing 34 also provides a means for deforming the right most end (as viewed in FIGS. 8 and 9) of plastic sleeve 33 which contacts wire braid outer conductor 4 inside outer casing 34, thereby pressing wire braid outer conductor 4 against outer casing 34, forming an electrical connection, for the purposes of shielding the central conductor 1. As will be appreciated by reference to FIG. 9, the end of post 32 (which is inserted between braid 4 and foil 3) is interior of outer casing 34, creating a partially blind insertion situation since the leading edge of post 32 is not easily observed during installation of connector 30 on a coaxial cable.